The invention relates to a method for the open-load diagnosis of a switching stage having a load connected in series with at least one switching transistor and to a device for carrying out the method.
In motor vehicle electronics, loads, for example, ignition coils or fuel injection valves, are increasingly being switched by electronic output stages, the output stage being disposed as near as possible to the load and being driven with small switching currents and voltages in order to avoid interference pulses from a driver switching stage that is disposed in a control unit and to which it is connected through cables (wiring harness). In the case of such a switching stage, an open-load diagnosis (testing for a line break) is intended to be carried out, where the intention is to fulfill the following boundary conditions:
a) the open-load diagnosis must be carried out online, i.e., during active operation, at least cyclically;
b) a large ratio (factor greater than 100) of maximum load current and open-load identification threshold (reference current) in the case of a small permissible voltage drop across the switching transistor (DMOS or MOSFET switch);
c) the open-load diagnosis must be carried out such that the permissible voltage drop across the output stage switch is not exceeded;
d) a high accuracy of the identification threshold with regard to process fluctuations when the switching stage is embodied as an integrated circuit;
e) the charge of the load capacitance at the output of the output stage switch (wiring harness capacitance) must be rapidly reversed when the switch is activated;
f) the solution must be suitable for cost-effective mass production. German Published, Non-Prosecuted Patent Application DE 40 20 187 C2, corresponding to U.S. Pat. No. 5,144,514 to Sekigawa et al., discloses a drive circuit for a load using a transistor device with a main transistor and a measuring transistor in parallel with the measuring transistor. A shunt resistor is connected in series with the measuring transistor. After the two transistors have been switched on, they are both switched off again if, after a predetermined delay time, the voltage dropped across the shunt resistor exceeds a predetermined reference value.
It is accordingly an object of the invention to provide a method and device for the open-load diagnosis of a switching stage that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that fulfills the above-mentioned boundary conditions.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method for an open-load diagnosis of a switching stage, including the steps of providing a switching stage having a load connected in series with at least one switching transistor, defining a predetermined first delay time period, starting a count of the first delay time period upon receipt of a control signal, waiting for the first delay time period to elapse, subsequently comparing an output voltage across the switching transistor with a predetermined first reference voltage, and terminating the open-load diagnosis and maintaining a driving of the switching stage until an end of the control signal where the output voltage is greater than the predetermined first reference voltage, and turning off the switching transistor and starting a count for a predetermined second delay time period where the output voltage is less than the predetermined first reference voltage, during the course of the second delay time period, comparing the output voltage with a predetermined second reference voltage, and, where the output voltage is greater than the predetermined second reference voltage terminating the open-load diagnosis, turning on the switching transistor again, and maintaining the driving of the switching stage until the end of the control signal, where the output voltage is less than the predetermined second reference voltage after the second delay time period has elapsed, comparing one of an output current flowing through the switching transistor and a voltage proportional to the output current with at least one of a predetermined reference current and a voltage proportional to the predetermined reference current, and terminating the open-load diagnosis and maintaining the driving of the switching stage until the end of the control signal where the output current is greater than the predetermined reference current, and effecting an open-load indication where the output current is less than the predetermined reference current, and repeating the open-load diagnosis from an end of the first delay time period until the end of the control signal after a predetermined waiting time period has elapsed.
In accordance with another mode of the invention, an optical indication identifying an open-load case through an indication signal is activated.
In accordance with a further mode of the invention, an entry identifying an open-load case is made in a diagnosis memory as a result of the indication signal.
With the objects of the invention in view, there is also provided a device for diagnosing an open-load of a switching stage, including a switchable buffer amplifier, a switching transistor having a drain-source path, the switching transistor to be controlled by control signals through the switchable buffer amplifier, a second transistor having a gate terminal, a source terminal, and a drain-source path connected in parallel with the drain-source path of the switching transistor, the second transistor to be controlled by the control signals, a third transistor having a gate terminal and a source terminal each connected to a respective one of the gate terminal and the source terminal of the second transistor, the third transistor to be controlled by the control signals, a reference current source connected to the drain terminal of the third transistor, a first comparator having two inputs and a first comparator output, one of the inputs connected to the drain terminal of the third transistor and another of the inputs connected to the drain terminal of the second transistor, the first comparator supplying an output signal at the first comparator output, a second comparator having two inputs and a second comparator output, one of the inputs connected to the drain terminal of the second transistor and another of the inputs receiving predetermined reference voltages, the second comparator supplying an output signal at the second comparator output, a control circuit connected to the buffer amplifier, to the second transistor, to the third transistor, to the first comparator, and to the second comparator, and the control circuit programmed to control the buffer amplifier, the switching transistor, the second transistor, and the third transistor and to predetermine the reference voltages dependent upon the control signals, the output signal of the first comparator,-and the output signal of the second comparator.
In accordance with an added feature of the invention, the switching transistor is a DMOS transistor.
In accordance with an additional feature of the invention, a ratio of current flowing through the switching transistor to current flowing through one of the group consisting of the second transistor and third transistor is greater than 100.
In accordance with yet another feature of the invention, the reference current source drives a reference current and the first comparator emits an output signal if a voltage drop caused across the third transistor by the reference current is greater than a voltage drop caused by current flowing through the second transistor.
In accordance with yet a further feature of the invention, the predetermined reference voltages include a first reference voltage and a second reference voltage, the switching transistor has an on state and an off state, and the second comparator emits an output signal if the first reference voltage is greater than an output voltage dropped across the switching transistor in the on state or the second reference voltage is greater than an output voltage dropped across the switching transistor in the off state.
In accordance with yet an added feature of the invention, the switching transistor is one of the group consisting of a low-side transistor and a high-side transistor.
In accordance with a concomitant feature of the invention, the switching transistor is a push-pull driver having both a low-side transistor and a high-side transistor and a switching stage is provided both for the low-side transistor and for the high-side transistor.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and device for the open-load diagnosis of a switching stage, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.